The present invention also concerns a method of processing silver-halide photographic products comprising a step in which the fixing or bleaching/fixing solution is treated continuously or discontinuously in order to eliminate the majority of the halide ions by means of nanofiltration membranes and in order to maintain an acceptable level of halide ions in the fixing solution during the entire processing period.
The basic method of obtaining a silver-halide photograph consists of exposing a silver-halide photographic product to actinic radiation which produces an image rendered visible by chemical processing. The fundamental steps of this treatment comprise, firstly, the treatment of the product with a developing solution in which a part of the silver halides is converted into metallic silver.
In order to obtain black and white images it is necessary to eliminate the undeveloped silver halides and in order to obtain a color image it is necessary to eliminate all the silver from the photographic product after the image has been formed. In black and white photography, the elimination of the undeveloped silver halides is achieved by dissolving them in a solvent for silver halides called a fixative and used in a fixing bath. In color photography, the elimination of the silver is achieved by oxidizing the metallic silver and dissolving the oxidized metallic silver and undeveloped silver halides. The oxidation of the metallic silver is achieved by means of a bleaching agent and the dissolution of the oxidized silver and the undeveloped silver halides is achieved by means of a fixing bath. The two operations can be combined by using a bleaching/fixing bath. In the remainder of the description, the term "fixing solution" will indiscriminately designate seasoned or used fixing or bleaching/fixing baths, that is to say those having already been used for fixing a certain quantity of photographic products.
It is extremely desirable to process the photographic products as quickly as possible. In particular, the fixing step should be as short as possible. During processing, reaction products accumulate in the fixing solutions. These products, largely dissolved silver and halide ions, slow down the fixing reaction and make the fixing solution less effective. To remedy this, the exhausted solution is reactivated by adding a regenerative solution after a certain quantity of photographic product has been processed. This does not prevent unwanted substances from accumulating in the processing solution, so that, finally, the activity of the solution can no longer be regenerated by adding fresh liquid. In practice, when the regenerative solution is introduced into the processing tank an equivalent quantity of exhausted solution is discharged by means of an overflow. In order to reduce the concentration of unwanted substances it is necessary to discharge a very large quantity of exhausted processing solution.
Furthermore, in order to produce the least possible pollution it is necessary to use quantities of chemical products which are as small as possible. One way of reducing the quantities of effluents discharged is to use low regeneration rates in the processing solutions, which makes it possible to reduce the volume of solution discharged via the overflow into the drainage system. If the regeneration rates are lowered, the concentrations of silver and halides increase again and the retarding effect is accentuated. Further, the problem of residual tints is posed, together with the fixing difficulties which results therefrom. Low regeneration rates can therefore be used only if unwanted substances are eliminated from the fixing solutions.
The degree of fixing can be improved and the volume of effluents reduced by eliminating the silver from used fixing solutions by chemical or electrochemical means. In practice, the concentration of silver ions is maintained at between 0.5 g/l and 1 g/l. However, these treatments do not eliminate the halide ions and in particular the iodide ions, which have a very considerable retarding effect. The increase in the concentration of bromide ions in the fixing solution does not pose any particular problem as the bromide ions have little influence on the kinetics of fixing.
Thus, if it were possible to eliminate the iodide ions from fixing solutions it would be possible to achieve faster fixing and this would prolong the life of the fixing solution whilst still using low regeneration rates.
The elimination of halide ions from the fixing solution is rendered difficult by the presence of other compounds in the solution, such as thiosulphate, sulphites and silver in chelated form. It is not desirable to eliminate the thiosulphate or sulphite ions, which are the active fixing agents. Unfortunately, numerous methods which could eliminate the iodide ions, such as oxidation, chelation and ion exchange interfere with these other anions. The sulphite and thiosulphate ions are easily oxidized. Numerous substances which precipitate or complex with the iodide ions also react with the thiosulphate ions. Anion exchange resins extract both iodide ions and thiosulphate ions.
Furthermore, the problem of eliminating iodide ions is rendered more difficult by the high concentration of compounds which are capable of interfering. The thiosulphate ions are generally present in a concentration of between 0.1 and 2.0 mol/l. The sulphite ions are present in a concentration of between 0.01 and 1 mol/l. The concentration of iodide ions should be kept below 0.05 mol/l, and preferably below 0.005 mol/l. This is why it is essential that the system of eliminating iodide ions be extremely selective, in particular with regard to thiosulphate ions.
U.S. Pat. No. 3,925,175 describes the elimination of silver and halide ions by causing the fixing solution to pass through the cathodic compartment of an electrolytic cell. The electrolytic cell comprises a semi-permeable membrane separating the anode and the cathode and also a solution of electro-active oxidizable substances in the anodic compartment. However, such semi-permeable membranes are expensive and easily clogged by the compounds in solution, which makes them ineffective for separation after a short time. In addition, this method requires electrical equipment and consumes electricity, which increases the cost and the complexity of the separation.
European Patent Application 0 348 532 describes a method in which the fixing solution is brought into contact with an ion exchange resin in order to accelerate the fixing of the photographic product containing silver iodide and to reduce the quantity of discharged effluents. However, either these resins eliminate ions other than the iodide ions, such as thiosulphate, sulphite and sequestered silver ions, or they cannot be used for eliminating the iodide ions in solutions containing numerous other anionic compounds.
German Patent DE-A-4 236 713 suggests the use of nanofiltration, or nanofiltration in combination with ultrafiltration, for separating the compounds dissolved in the washing water of the bleaching/fixing step of photographic processing. A first step, at a high flow rate and low concentration, makes it possible to recover the clean water, which can be recycled into the rinsing tanks. A second step, at a low flow rate and high concentration, enables the fixing compounds present to be reconcentrated in the washing water so as to produce an aqueous photographic solution which can be used for fixing or bleaching/fixing.
U.S. Pat. No. 5,219,717 describes a method for selectively eliminating the iodide ions in fixing and bleaching/fixing baths in which an anionic surfactant, a medium absorbing the iodide and a polymer are used, the anionic surfactant having the opposite charge to that of the polymer. For example, the absorbent medium can be silver bromide and the polymer a copolymer of methacrylate, methacrylamide, acrylate or acrylamide. This very selective method enables the iodide ions to be eliminated without changing the thiosulphate concentration. However, the method is difficult to implement as a polymer support band covered with the compound absorbing the iodide ions and with the surfactant must be made to circulate in the vessel containing the fixing solution.
EP-A-0 407 979 suggests the use of a technology involving reverse osmosis for regenerating and recycling washing water or fixing solutions.
U.S. Ser. No. 08/691,198, filed Aug. 7, 1996; entitled "Method and Device for the Selective Extraction of Halide Ions from Photographic Baths", by Didier J. Martin, Jean-Francois Diaz, and Christian G. Guizard, describes the regenerating and of a fixing solution using two nanofiltration steps. In a first step a preliminary separation is effected by subjecting the fixing or bleaching/fixing solution to a first step of nanofiltration through a tubular membrane used in concentration mode. The permeate produced in this first step is subjected to a second step of nanofiltration through a spiral membrane used in diafiltration mode.
The approach described in the application mentioned above, even if it partly resolves the problems outlined above, is not completely satisfactory. This is because it allows only around 50% of the halide ions to be extracted from the bleaching or bleaching/fixing solution. In addition, it involves the use of two different types of nanofiltration membrane, which substantially increases the cost of implementing the method. Finally, according to this method, the loss of concentration of thiosulphate is in any case around 10%. In addition, it is not possible to concentrate the fixing solution with regard to thiosulphate ions.
This is why it is desirable to have available a method which is simple to implement and which enables the problems discussed above with reference to the prior art to be substantially resolved.